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Henderson, DJ, Rushbrook, JL, Stewart, TD et al. (1 more author) (2016) What Are the Biomechanical Effects of Half-pin and Fine-wire Configurations on Fracture Site Movementin Circular Frames? Clinical Orthopaedics and Related Research, 474 (1). pp. 1041-1049. ISSN 0009-921X

https://doi.org/10.1007/s11999-015-4652-8

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What Are the Biomechanical Effects of Half-pin and Fine-wire Configuration on Fracture Site Movement in Circular Frames? Running title: Frame Configuration and Fracture Motion Daniel J. Henderson FRCS (Orth), Jeremy L. Rushbrook FRCS (Orth), Todd D. Stewart PhD, Paul J. Harwood FRCS (Orth) D. J. Henderson, J. L. Rushbrook, P. J. Harwood Department of Orthopaedics, Leeds General Infirmary, Leeds, UK D. J. Henderson, J. L. Rushbrook, T. D. Stewart Institute of Medical and Biological Engineering, School of Mechanical Engineering, University of Leeds, Leeds, UK Each author certifies that he or she, or a member of his or her immediate family, has no funding or commercial associations (eg, consultancies, stock ownership, equity interest, patent/licensing arrangements, etc) that might pose a conflict of interest in connection with the submitted article.

All ICMJE Conflict of Interest Forms for authors and Clinical Orthopaedics and Related Research® editors and board members are on file with the publication and can be viewed on request. Clinical Orthopaedics and Related Research® neither advocates nor endorses the use of any treatment, drug, or device. Readers are encouraged to always seek additional information, including FDA-approval status, of any drug or device prior to clinical use. This work was performed in the laboratories of the Institute of Medical and Biological Engineering at the University of Leeds, Leeds, UK. D. J. Henderson Department of Orthopaedics Leeds General Infirmary Great George Street Leeds, LS1 3EX, UK email: Daniel.Henderson@nhs.net

Abstract

Background Fine-wire circular frame (Ilizarov) fixators are hypothesized to generate

favorable biomechanical conditions for fracture healing, allowing axial micromotion while

limiting interfragmentary shear. Use of half-pins increases fixation options and may improve

patient comfort by reducing muscle irritation, but they are thought to induce interfragmentary

shear, converting beam-to-cantilever loading. Little evidence exists regarding the magnitude

and type of strain in such constructs during weightbearing.

Questions/purposes This biomechanical study was designed to investigate the levels of

interfragmentary strain occurring during physiologic loading of an Ilizarov frame and the

effect on this of substituting half-pins for fine wires.

Methods The “control” construct comprised of a four-ring all fine-wire construct with plain

wires at 90 crossing angles in an entirely unstable acrylic pipe synthetic fracture model.

Various configurations, substituting half-pins for wires, were tested under levels of axial

compression, cantilever bending, and rotational torque simulating loading during gait. In total

three frames were tested for each of five constructs, from all fine-wire to all half-pin.

Results Substitution of half-pins for wires was associated with increased overall construct

rigidity and reduced planar interfragmentary motion, most markedly between all-wire and all-

pin frames (axial: 5.9 mm +/-0.7 vs 4.2 mm +/-0.1, mean difference, 1.7 mm, 95% CI, 0.8-

2.6 mm, p < 0.001; torsional: 1.4% +/-0.1 vs 1.1% +/-0.0 rotational shear, mean difference,

0.3%, 95% CI, 0.1%-0.5%, p = 0.011; bending: 7.5° +/-0.1 vs 3.4° +/-0.1, mean difference, -

4.1o, 95% CI, -4.4 o to -3.8 o, p < 0.001). Although greater transverse shear strain was

observed during axial loading (0.4% +/-0.2 vs 1.9% +/-0.1, mean difference, 1.4%, 95% CI,

1.0%-1.9%, p < 0.001), this increase is unlikely to be of clinical relevance given the current

body of evidence showing bone healing under shear strains of up to 25%. The greatest

transverse shear was observed under bending loads in all fine-wire frames, approaching 30%

(29% +/-1.9). This was reduced to 8% (+/-0.2) by incorporation of sagittal plane half-pins

and 7% (+/-0.2) in all half-pin frames (mean difference, -13.2% and -14.0%, 95% CI, -16.6%

to 9.7% and -17.5% to -10.6%, both p < 0.001).

Conclusions Appropriate use of half-pins may reduce levels of shear strain on physiologic

loading of circular frames without otherwise altering the fracture site mechanical

environment at levels likely to be clinically important. Given the limitations of a

biomechanical study using a symmetric and uniform synthetic bone model, further clinical

studies are needed to confirm these conclusions in vivo.

Clinical Relevance The findings of this study add to the overall understanding of the

mechanics of circular frame fixation and, if replicated in the clinical setting, may be applied

to the preoperative planning of frame treatment, particularly in unstable fractures or bone

transport where control of shear strain is a priority.

Introduction

The Ilizarov method is clinically established in fracture management, limb salvage, and

deformity correction [10,18]. Research has been conducted to determine the basic

mechanisms underlying this technique and is particularly relevant as insights are gained in

the biomechanical requirements for and biology of bone healing. The mechanical

environment at a fracture or osteotomy site is a principal factor in achieving union with

certain levels of mechanical stimulus required to initiate and propagate secondary bone

healing [14]. It generally is accepted that certain levels of axial motion help stimulate

healing, whereas shear may delay or prevent it, potentially resulting in nonunion. Studies of

this subject, however, while confirming the beneficial effects of axial micromotion, often fail

to agree on the detrimental effect of shear strain, with Bishop et al. [2] reporting improved

fracture healing with 25% shear strain over rigid fixation, and the topic clearly is not yet fully

understood [8,16]. The Ilizarov method, through variation of construct and fixation elements,

allows powerful manipulation of fracture site mechanics, and it generally is considered that

correct application of fine-wire circular frame fixation methods can help control the

mechanical environment at a fracture site in ways conducive to bone healing [5, 9]. Crossed,

tensioned fine wires appear to generate a self-stiffening, beam-loading environment,

producing favorable levels of axial micromotion while limiting shear as patients bear weight

[5, 20, 24].

The main disadvantage of fine wires is the relatively narrow anatomic corridors in which they

can be placed to minimize the risk of neurovascular damage [6]. By their nature, these wires

often transfix muscle and irritate tendons, leading to pain, loss of mobility, and potentially

increasing the risk of pin site infection. Half-pins, by contrast, can be inserted safely through

the majority of the tibia in an arc from anterior to direct medial, across the subcutaneous face

of the tibia [6]. This opens wider avenues of fixation and largely avoids muscle transfixation.

Some surgeons advocate the use of half-pins on the basis that this will increase patient

comfort and compliance [4, 13]; however, the introduction of half-pins in a construct

potentially leads to cantilever bending during loading, which is theorized to increase shear at

the fracture [13]. For this reason some surgeons avoid the use of half-pins wherever possible.

Despite these opposing viewpoints, there is little experimental evidence investigating

interfragmentary strain that occurs in circular frames using fine wires or half-pins and none to

our knowledge addressing the complex loading forces placed on the framed limb during gait.

This biomechanical study therefore was designed to investigate the following research

questions: (1) What type and magnitude of interfragmentary strain occurs on loading of a

simulated tibial fracture in an Ilizarov fine-wire frame under physiologic levels of axial

loading, axial torsion, and bending loads? (2) How is interfragmentary strain altered by

substitution of half-pins for fine wires under these conditions? From this we hoped to

determine the ideal frame construct for optimizing the mechanical environment under

physiologic loading conditions.

Materials and Methods

An experimental biomechanical study was designed using an acrylic tube with an outer

diameter of 32 mm and a 4-mm wall (Clear Plastic Supplies, Chesterfield, UK), as a

mechanical substitute for bone. This symmetric, uniform model was selected to minimize

variability between testing cycles attributable to minor variations in wire and pin placement

in more anatomic models, and to allow focus to be kept on the effect of altering bone fixation

elements alone. This methodology is in keeping with previous studies [21-24, 26, 27].

Five construct configurations were sequentially tested: all fine-wire frame; all half-pin

frame; fine wires with one half-pin either side of the fracture in the same plane; one wire and

one half-pin at each ring with pins at 90° on adjacent rings; and one wire and one half-pin at

each ring with all half-pins in the same plane (Fig. 1). The all fine-wire fixation model was

tested with bending load in the plane perpendicular to the wires and at 45.

Three identical four-ring frames were tested for each configuration with 50 mm between

common segment rings and 175 mm across the interfragmentary segment, constructed

according to the manufacturer’s specifications, using 155-mm aluminum Taylor Spatial

FrameTM rings (Smith & Nephew, Inc, Memphis, TN, USA) with four 6-mm threaded rods

between each ring.

The plastic simulated bone segments were mounted with a 20-mm simulated fracture gap to

prevent cylinder end apposition during loading, mimicking an entirely unstable fracture or

corticotomy. Fixation was achieved using previously unused clinical standard 1.8-mm

smooth wires, tensioned to 130 kg, or predrilled 5-mm self-tapping half-pins. To maximize

reproducibility of the study, to isolate the effect of half-pin use, and in keeping with previous

studies, wires and pins were placed at theoretically “ideal” crossing angles of 90° with the

bone substitute mounted centrally in each ring [5, 23, 27]. Plain wires were used throughout.

Constructs were mounted in the testing apparatus (Tinius Olsen H25K-S UTM; Tinius Olsen

Inc, Horsham, PA, USA; and uniaxial manual torsion testing machine; University of Leeds,

Leeds, UK) using bespoke mounting jigs, allowing rigid fixation of each end of the bone

substitute to the apparatus (Fig. 2).

Each frame was tested separately in axial loading, AP cantilever bending, and axial torsion.

Three preconditioning cycles of full loading were applied before data collection at loading

rates of 6 mm/minute/1.01°/minute. Loading was applied to a maximum of 700 N axially, 20

Nm bending, and 20 Nm torsion. Previous studies [7] showed that by 30 days postoperatively

the majority of patients with tibial circular fixation are placing full body weight through their

affected limb, and moreover, clinical studies [28], using instrumented frames, showed that in

severely comminuted fractures up to 70% of ground reaction force is supported by the frame

as axial load, 2.5% (Nm/N) as bending load, and 0.75% (Nm/N) as torsion. On the basis of an

80-kg patient therefore, physiologic loading was considered to consist of 500-N axial

loading, 20-Nm bending, and 6-Nm torsional load, rounded to 5-Nm for testing.

Strain was recorded through measurement of bone segment displacement during loading

using Digimatic linear variable differential transformers, with 0.01 mm resolution (Mitutoyo

(UK) Ltd, Andover, UK), positioned at the fracture site to record interfragmentary movement

in three planes (Fig. 3).

Data were collected, collated, and tabulated using Microsoft Excel® (Microsoft Corporation,

Redmond, WA, USA), with data sets subsequently transferred for graph plotting and

statistical analysis to Graph Pad Prism® (Version 6; GraphPad Software, Inc, La Jolla, CA,

USA). Load deformation curves were created for each construct and loading regime for the

full range of loading. Maximum interfragmentary displacement at physiologic loading then

was determined. Displacement in the plane of applied load (mm, degrees bending, or degrees

rotation) was plotted against composite transverse plane shear strain (mm and % strain).

Statistical analysis was done to determine whether there was any significant difference in

interfragmentary movement between the tested constructs. The data met the assumptions for

parametric testing using the D’Agostino-Pearson omnibus test and QQ-plot analysis and

therefore ANOVA testing for statistical significance was done with post hoc analysis using

Tukey’s method. A probability value less than 0.05 was considered statistically significant

throughout.

Results

Axial Loading

Under axial loading the all fine-wire construct underwent nonlinear axial deformation as load

increased. At 500-N loading, a mean axial interfragmentary motion of 5.9 mm compression

was observed with minimal transverse linear shear of 0.4% (Fig. 4). By contrast, the all half-

pin construct was more rigid and performed more consistently with lower mean

interfragmentary compression of 4.2 mm (mean difference, 1.7 mm, 95% CI, 0.8-2.6 mm, p <

0.001) but greater transverse shear strain of 1.9% (mean difference, 1.4%, 95% CI, 1.0%-

1.9%, p < 0.001) (Fig.4). The hybrid pin/wire constructs showed axial interfragmentary

motion that was not different from those of the wire frames at 5.9 mm, 5.3 mm, and 5.2 mm

for constructs 1, 2, and 3, respectively (mean differences, -0.1 mm, 0.6 mm, 0.7 mm, 95% CI,

-1.0 to 0.7 mm, -0.3 to 1.5 mm, -0.2 to 1.6 mm, p = 0.983, p = 0.239, and p = 0.147).

However, the hybrid constructs showed greater shear strains: 1.4%, 1.0%, and 2.0%,

respectively, compared with the wire frame (mean differences, 1.0%, 0.6%, 1.6%, 95% CI,

0.5%-1.4%, 0.1%-1.1%, 1.1%-2.1%, p < 0.001, p = 0.010, and p < 0.001) (Table 1).

Axial Torsion

Under torsional loading of 5 Nm, a mean interfragmentary rotational shear strain of 1.4% was

observed in the wire frame with negligible transverse shear (Fig. 5). The all half-pin frames

were more rigid, producing only 1.1% rotational shear strain at 5 Nm (mean difference, 0.3

Nm, 95% CI, 0.1-0.5 Nm, p = 0.011) but with no difference observed in lateral translation at

the fracture site, at 0.4% (mean difference, 0.4%, 95% CI, -0.9% to 1.6%, p = 0.851). Hybrid

construct 3, with one half-pin and one wire on each ring, also showed increased rigidity

compared with the wire frames, producing 1.1% rotational shear strain (mean difference,

0.3%, 95% CI, 0.1%-0.5%, p = 0.011), but with increased transverse shear of 1.8% (mean

difference, 1.8%, 95% CI, 0.5%-3.0%, p = 0.006) By contrast, hybrid construct 2, also with

one half-pin and one wire per ring, but orientated perpendicularly on adjacent rings, showed

no difference in rigidity from the all wire-frames, with 1.2% rotational shear (mean

difference, 0.2%, 95% CI, 0.0%-0.4%, p = 0.107), nor any difference in translational shear, at

0.2% (mean difference, -0.2%, 95% CI, -1.4% to 1.0%, p = 0.982). Finally, hybrid construct

1, with only one half-pin per bone segment, although showing no difference in rigidity to the

wire frames, with 1.6% rotational shear (mean difference, -0.2%, 95% CI, -0.4% to 0.1%, p =

0.166), showed the most marked increase in translational shear strain, of 3.3% (mean

difference, -3.3%, 95% CI, -4.5% to 2.0%, p < 0.001) (Table 1).

Bending Loading

The most marked variations in mechanical behavior were observed under bending loads.

Bending load of 20 Nm placed on the all-wire frames, with wires in the coronal and sagittal

planes, produced a mean of 7.5° interfragmentary angulation and 21% transverse shear (Fig.

6). Testing with the wires at 45° to the coronal and sagittal planes only increased this

displacement, producing 8.5° angulation (mean difference, 1.0, 95% CI, 0.7°-1.3°, p < 0.001)

and greater translation at 29% (mean difference, 8.0°, 95% CI, 4.5°-11.4°, p < 0.001). In

contrast, the all-pin frames showed 3.4° interfragmentary angulation with 7% transverse

shear, both lower than seen in the all-wire frames (mean differences, -4.1° and -14.0°, 95%

CI, -4.4°to -3.8° and -17.5° to -10.6°, both p < 0.001). Testing of the hybrid constructs with

the half-pins placed in the coronal plane or, in the case of construct 2, with the pins closest to

the fracture in the coronal plane produced results somewhere between the two, with 4.6°,

5.1°, and 4.6° angular displacement seen in constructs 1, 2, and 3 (vs all-wire: mean

differences, -2.9°, -2.4°, and -2.9°, 95% CI, -3.1° to -2.5°, -2.7° to -2.1°, and -3.2° to 2.6°, all

p < 0.001), together with 20%, 20%, and 16% transverse shear (vs all-wire: mean differences,

-1.2%, -1.5%, and -5.6%, 95% CI, -4.7% to 2.2%, -5.0% to 1.9%, and -9.0% to -2.2%, p =

0.934, p = 0.819, and p < 0.001), respectively. In contrast, placing the half-pins in the

sagittal plane (parallel to the plane of applied bending load) reduced transverse shear for all

three constructs (vs all-wire: 16%, 13%, 8%; mean differences, -5.4%, -8.0%, -13.2%; 95%

CI, -8.8% to -1.9%, -11.5% to -4.6%, -16.6% to -9.7%; p = 0.01, p < 0.001, p < 0.001) to

levels lower than in the all-wire frames, but most markedly so in construct 3 where transverse

shear was reduced to 8% (mean difference, -13.2%, 95% CI, -16.6% to 9.7%, p < 0.001)

(Table 1).

Discussion

The use of circular fixators is well established, with widespread anecdotal and clinical reports

of positive outcomes in various challenging cases [10,18]. As an entirely modular system,

good clinical outcomes rely on an appropriate understanding of how best to apply the frame

components, tailored to each patient, to produce a construct which will sufficiently control

the mechanical environment at the fracture or osteotomy site to allow bone healing [27, 29].

Developments on the original Ilizarov method of fine wire fixation have introduced elements,

such as threaded half-pins, which allow more versatility in frame application and with

reduced muscle transfixion. Although purported to improve patient comfort and tolerance,

this has raised concerns that, by altering the mechanics of the frame construct through

cantilever bending and introducing increased shear, such elements may have a detrimental

effect on the biomechanical environment at a fracture or osteotomy site [4, 6, 13]. Our study

was designed to investigate levels of interfragmentary strain occurring in a completely

unstable experimental fracture or osteotomy model stabilized by various configurations of a

circular frame. The key research questions were: what interfragmentary motion occurs in a

fine-wire circular frame under physiologic loading conditions and how is this motion affected

by substitution of half-pins for fine wires in various configurations? By consideration of

these results along with what is known about fracture healing environments and fixation

approaches [2, 8, 11, 17, 19, 30, 31], we sought to formulate some recommendations

regarding favorable frame configurations for fracture healing.

This study has limitations reflecting its experimental nature and methodology, which must be

taken into consideration when interpreting the results. Attempts were made during design to

limit the number of variables between frames to focus on the specific elements under

investigation and improve reproducibility between constructs and testing cycles. This, in

addition to the absence of surrounding soft tissues, means that this study is not able or

intended to replicate the precise movement of a fracture in vivo, but instead allows analysis

of the mechanical effect of specific alterations in frame construction under controlled

conditions and relevant comparisons drawn. The acrylic pipe synthetic bone model, rather

than being anatomically contoured, is symmetric and uniform, allowing reproducible

application and loading of bone fixation elements which would be impossible in cadaveric

specimens. Its hollow nature allows “bicortical” fixation, replicating stress distributions on

wires and half-pins identified in a previous experimental study [3]. The experimental frame

constructs, with idealized fine wire and pin crossing angles, are not usually possible in the

clinical setting, and therefore should be taken in the light of previous work investigating the

effect of altered crossing angles on frame mechanical behavior [21, 23]. Moreover, use of

synthetic acrylic tubes and idealized wire placement, rather than bone-shaped models and

anatomic fixation placement, minimized variability resulting from minor differences in wire

positioning or plane of loading, allowing greater confidence in the reproducibility of results

between testing cycles, and the precision achieved during three loading cycles for each frame

configuration.

Additionally, the simplified loading regimens applied here, with axial, bending, and torsional

loads applied separately, does not replicate true multidirectional dynamic loading during gait

which would be extremely difficult to replicate in vitro, but does allow for better comparison

of frame constructs under each mode of loading and helps understanding of the strain induced

by each axis of load.

Our study is not intended to replicate the in vivo scenario or specifically analyze the

performance of each construct in isolation, but to allow comparison of the effect modifying

frame configuration has on mechanical behavior and interfragmentary strain.

Axial Loading

The pattern of observed interfragmentary strain and construct rigidity under axial loading in

an all fine-wire frame correlated closely with previously published data ; tensioned fine wires

behaved in a “self-stiffening” nonlinear manner with minimal shear strain observed [1, 13,

32]. Likewise, substitution of half-pins for fine wires under axial load also produced results

in keeping with those described by Gessmann et al. [13], with reduced axial interfragmentary

strain and increased shear strain generated by cantilever bending [32]. Despite this increase

in axial stiffness, the half-pin construct still generated 4 mm of mean interfragmentary

compression, and with only 1.9% shear, falling well within the greater than 0.5 mm axial

strain and up to 25% shear strain reported to induce good bone healing [2, 8, 12, 19].

Observed axial interfragmentary strains in the hybrid constructs were not different from those

in the all fine-wire construct; however, transverse plane shear strain, similarly to the half-pin

construct, was greater, likely reflecting a degree of cantilever bend occurring in the system.

The magnitude of this strain in all cases was at levels that appear unlikely to be of clinical

importance.

Axial Torsion

Torsional loading in fine wire frames produced little interfragmentary transverse plane

rotational shear, which was reduced further by the introduction of half-pins. Half-pins,

however, did produce increased levels of translational interfragmentary shear strain,

particularly with use of a single pin in each bone segments (construct 1), albeit at levels

below those likely to be detrimental to bone healing. We found no published studies

investigating the behavior of ring fixators under torsional loading against which to compare

these findings. Given that clear differences in behavior were seen with substitution of half

pins on axial torsional loading, future studies would benefit from investigating this effect

further in a more clinical construct or anatomically correct model.

Bending Loading

Few comparative data exist regarding the behavior of fractures treated with circular frames

under bending loads, the only identified comparable work [32] showed little shear strain or

angular deformation on loading of a fine wire frame and a hybrid device similar to construct

3 in the current study. This is in contrast to the high levels of shear strain we observed under

physiologic bending loads, a difference which may be attributable to numerous factors: Yang

et al. [32] applied four-point bending, centered at the fracture site, producing symmetric

deformation and potentially less interfragmentary shear than the more physiologic cantilever

loading we used. Furthermore lower forces were applied at 10 Nm torque and a solid

synthetic bone analog was used with larger wire contact than cannulated pipe. Finally, the

frames we tested were constructed with a larger interfragmentary distance and greater

distance between interfragmentary rings than those of Yang et al. [32], both factors intended

to emphasize any observed shear strain for comparative analysis. The mechanism of the large

interfragmentary shear strains observed in the current study was attributable to a stick-shift

mechanism of movement of the bone model along the tension fine-wires. Even with

reportedly ideal 90° wire crossing angles, flexion of the perpendicular wire allowed

displacement of the bone model along the wire parallel to the plane of loading. The addition

of half-pins in the plane of bending loading resulted in increased construct rigidity and

reduced transverse shear, in keeping with similar findings and conclusions by Yang et al.

[32]. This aspect of frame biomechanics warrants further investigation in future studies and

particularly in more directly clinically relevant constructs. Although not universally used by

frame surgeons, the effect of olive wires in resisting the observed slippage of the bone on fine

wires would be of particular interest.

Clinical Relevance

In relating these results to real-world patients, it is important to reflect on not only the

existing body of biomechanical evidence, but also the long and well-documented history of

successful treatment of a wide variety of fractures and deformities using the Ilizarov method

and fine-wire fixators [10, 15, 18]. On this basis, it seems fair to assume that properly applied

fine-wire frames produce an environment at a fracture site compatible with bone healing, an

assumption largely supported by the results we discussed. The high levels of shear strain

showed in fine-wire frames under bending loads alone fall outside what might be considered

conditions necessary for bone healing [2, 7, 8, 12]. The current study does not aim to

simulate an exact clinical scenario, but to allow comparison between constructs, and so these

high levels of shear likely represent an experimentally exaggerated magnitude of behavior,

attributable to the factors discussed previously, rather than a true in vivo picture. With this

caveat however, the comparative findings of this study in the relative effect of substituting

fine-wires for half-pins, corroborated by the limited evidence, may be translated to the

clinical setting, as well as providing a basis for further research in this area.

Overall, although many of the findings of our study are concordant with those published

previously, numerous previously undescribed mechanical behaviors were observed [1, 13,

32]. We did not see the purported clinically significant increase in shear strain, or reduction

in axial micromotion, on loading of frames incorporating half-pins. Under simulated

physiologic levels of applied axial and torsional load, use of half-pins was seen to produce a

more rigid construct, and with a small increase in translational shear at the fracture site,

however the clinical importance of these changes in axial and shear strain is doubtful given

that magnitudes still fall within levels supported by current evidence as promoting bone

healing [2, 8, 25]. If single half-pins are used in a segment however, they appear to act as

pivot points; this induces greater levels of rotational shear at the fracture gap and should

probably be avoided if possible. Under physiologic bending loads, high levels of shear strain

were observed in fine-wire frames which (although these are unlikely to be representative of

true in-vivo levels of shear) illustrates the potential for construct instability during

weightbearing. This may be a particular concern in cases of gross fracture comminution, de-

gloved open injuries, or bone transport where lack of bone apposition, support, and wide ring

spacing further reduces fracture stability. It is with these scenarios that the findings of this

study show that symmetrically placed sagittal near and far half-pins, on rings on either side of

the fracture, effectively increase construct stiffness and reduce shear strain associated with

bending loads on heel strike during gait beyond that seen with smooth tensioned fine-wire

frames [7, 8, 31]. Given the potential advantages in terms of reduced soft tissue transfixion

and patient comfort, these findings may allow surgeons more confidence with the use of half-

pins under such circumstances although these results need to be corroborated in a clinical

model.

Acknowledgments

We thank Smith & Nephew Advanced Surgical Devices (Huntingdon, UK), and Guy Crosby

in particular, for donation of the circular frame materials for testing.

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Figures

Fig. 1 The five frame configurations tested, wires only, wires and pins construct 1, wires and

pins construct 2, wires and pins construct 3, and pins only, are shown.

Fig. 2 An all half-pin frame mounted in the axial testing apparatus (Tinius Olsen H25K-S

UTM; Tinius Olsen Inc, Horsham, PA, USA) is shown.

Fig. 3A-C The planes of displacement, (A) axial loading (axial displacement = x-y, linear

transverse shear = z), (B) bending loading (angular deformation = g, linear transverse shear =

z) and (C) torsional loading (rotational deformation = g, linear transverse shear = z) are

shown.

Table 1. Results table for construct testing under axial, torsional, and bending loads Values are mean SD.

Mode of loading Fracture site movement All wires All Half-pins Construct 1 Construct 2 Construct 3

Axial loading 500 N Axial displacement (mm) 5.9 0.7 4.2 0.1 6.0 0.0 5.3 0.2 5.2 0.1 Translational shear (mm) 0.1 0.1 0.6 0.0 0.4 0.1 0.3 0.1 0.6 0.0 Translational shear strain 0.4% 0.2% 1.9% 0.1% 1.4% 0.2% 1.0% 0.2% 2.0% 0.1%

Torsional loading 5 Nm Torsional shear strain 1.4% 0.1% 1.1% 0.0% 1.6% 0.1% 1.2% 0.1% 1.1% 0.1% Translational shear (mm) 0.0 0.0 0.1 0.1 1.0 0.1 0.1 0.0 0.5 0.3 Translational shear strain 0.0% 0.0% 0.4% 0.2% 3.2% 0.4% 0.2% 0.1% 1.8% 0.9%

Bending loading 20 Nm Hybrid pins in sagittal plane

Bending displacement (°) 7.5 0.1 3.4 0.1 6.2 0.2 5.6 0.1 3.2 0.0 Translational shear (mm) 6.4 0.7 2.2 0.1 4.8 0.5 4.0 0.1 2.5 0.1 Translational shear strain 21% 2.2% 7% 0.2% 16% 1.5% 13% 0.4% 8% 0.2%

Wires 45° and hybrid pins coronal

Bending displacement (°) 8.5 0.1 4.6 0.1 5.1 0.1 4.6 0.0 Translational shear (mm) 8.8 0.6 6.1 0.3 6.0 0.3 4.7 0.0 Translational shear strain 29% 1.9% 20% 1.1% 19% 0.9% 15% 0.1%

Fig. 4 The maximum axial and shear strains observed under 500-N axial loading are shown.

Fig. 5 Maximum rotational and transverse shear strains observed under 5-Nm torsional load

are shown.

Fig. 6 The maximum bending and shear strains observed under 20-Nm bending load are

shown.